(1) Field of the Invention
The present invention relates to semiconductor devices and more particularly to a method of forming a split-gate flash memory cell in order to reduce the size of the cell and at the same time increase the coupling ratio between the floating gate and the source for improved program speed.
(2) Description of the Related Art
A conventional split-gate flash memory device is characterized by its split-gate side (between the control gate and the drain) and the stacked-side (between the floating gate and the source) and by a coupling ratio between the floating gate and the source. As is known, the coupling ratio affects the program speed, that is, the larger the coupling ratio, the faster is the programming speed, and is not a fixed value by virtue of the variability of the channel length and hence that of the overlap between the floating gate and the source. Usually, if channel length is increased through greater lateral diffusion in the source region, punchthrough occurs due to excessive current well below the threshold voltage. It is shown in the present invention that the coupling ratio can be increased without increasing the channel length, but by incorporating side-wall coupling of poly on source line and, at the same time, alleviating the punchthrough and junction break-down of source region by sharing gate voltage with the poly on source line.
Over the years, numerous improvements in the performance as well as in the size of memory devices have been made by varying the simple, basic one-transistor memory cell, which contains one transistor and one capacitor. The variations consist of different methods of forming capacitors, with single, double or triple layers of polysilicon, and different materials for the word and bit lines. In general, memory devices include electrically erasable and electrically programmable read-only memories (EEPROMs) of flash electrically erasable and electrically programmable read-only memories (flash EEPROMs). Many types of memory cells for EEPROMs or flash EEPROMs may have source and drains regions that are aligned to a floating gate or aligned to spacers. When the source and drain regions are aligned to the floating gate, a gate electrode for a select transistor is separate from the control gate electrode of the floating gate transistor. Separate select and control gates increase the size of the memory cell. If the source and drain regions are aligned to a spacer formed after the floating gate is formed, the floating gate typically does not overlie portions of the source and drain regions. Programming and erasing performance is degraded by the offset between the floating gate and source and drain regions.
Most conventional flash-EEPROM cells use a double-polysilicon (poly) structure of which the well known split-gate cell is shown in FIG. 1A. Here, two MOS transistors share a source (25). Each transistor is formed on a semiconductor substrate (10) having a first doped region (20), a second doped region (25), a channel region (23), a gate oxide (30), a floating gate (40), intergate dielectric layer (50) and control gate (60). Substrate (10) and channel region (23) have a first conductivity type, and the first (20) and second (25) doped regions have a second conductivity type that is opposite the first conductivity type.
As seen in FIG. 1A, the first doped region, (20), lies within the substrate. The second doped region, (25), also lies within substrate (10) and is spaced apart form the first doped region (20). Channel region (23) lies within substrate (10) and between first (20) and second (25) doped regions. Gate oxide layer (30) overlies substrate (10). Floating gate (40), to which there is no direct electrical connection, and which overlies substrate (10), is separated from substrate (10) by a thin layer of gate oxide (30) while control gate (60), to which there is direct electrical connection, is generally positioned over the floating gate with intergate oxide (50) therebetween.
In the structure shown in FIG. 1A, control gate (60) overlaps the channel region, (23 under the floating gate, (40). This structure is needed because when the cell is erased, it leaves a positive charge on the floating gate. As a result, the channel under the floating gate becomes inverted. The series MOS transistor (formed by the control gate over the channel region) is needed in order to prevent current flow from control gate to floating gate. The length of the transistor, that is the overlap of the control gate over the channel region (23) determines the cell performance. Furthermore, edges (41), (43) can affect the programming of the cell by the source size and hot electron injection through the intergate dielectric layer (50) at such edges. Hot electron injection is further affected by, what is called, gate bird""s beak (43) that is formed in conventional cells. On the other hand, it will be known to those skilled in the art that corners such as (41) can affect the source coupling ratio also. Any such adverse effects attributable to the source size can be alleviated as disclosed later in the embodiments of this invention.
To program the transistor shown in FIG. 1A, charge is transferred from substrate (10) through gate oxide (30) and is stored on floating gate (40) of the transistor. The amount of charge is set to one of two levels to indicate whether the cell has been programmed xe2x80x9conxe2x80x9d or xe2x80x9coff.xe2x80x9d xe2x80x9cReadingxe2x80x9d of the cell""s state is accomplished by applying appropriate voltages to the cell source (25) and drain (20), and to control gate (60), and then sensing the amount of charge on floating gate (40). To erase the contents of the cell, the programming process is reversed, namely, charges are removed from the floating gate by transferring them back to the substrate through the gate oxide.
This programming and erasing of an EEPROM is accomplished electrically and in-circuit by using Fowler-Nordheim tunneling as is well known in prior art. Basically, a sufficiently high voltage is applied to the control gate and drain while the source is grounded to create a flow of electrons in the channel region in the substrate. Some of these electrons gain enough energy to transfer from the substrate to the floating gate through the thin gate oxide layer by means of Fowler-Nordheim tunneling. The tunneling is achieved by raising the voltage level on the control gate to a sufficiently high value of about 12 volts. As the electronic charge builds up on the floating gate, the electric field is reduced, which reduces the electron flow. When, finally, the high voltage is removed, the floating gate remains charged to a value larger than the threshold voltage of a logic high that would turn it on. Thus, even when a logic high is applied to the control gate, the EEPROM remains off. Since tunneling process is reversible, the floating gate can be erased by grounding the control gate and raising the drain voltage, thereby causing the stored charge on the floating gate to flow back to the substrate. Of importance in the tunneling region is the quality and the thinness of the tunneling oxide separating the floating gate from the substrate.
In the conventional memory cell shown in FIG. 1A, word lines (not shown) are connected to control gate (60) of the MOS transistor, while the length of the MOS transistor itself is defined by the source (25) drain (20) n+ regions shown in the same Figure. As is well known by those skilled in the art, the transistor channel is defined by masking the n+ regions. However, the channel length of the transistor varies depending upon the alignment of the floating gate (40) with the source and drain regions. This introduces significant variations in cell performance from die to die and from wafer to wafer. Furthermore, the uncertainty in the final position of the n+ regions causes variations in the series resistance of the bit lines connected to those regions, and hence additional variation in the cell performance.
Prior art contains different methods for fabricating different split-gate memory cells. In U.S. Pat. No. 5,480,821 by Chang, a method of fabricating source-coupling, split-gate, virtual ground flash EEPROM array is proposed, while in U.S. Pat. No. 5,412,238 by the same inventor a structure of the same split-gate memory cell is shown. The floating gate of each cell is formed over a first portion of the substrate channel region and is separated from the channel region by a layer of floating gate oxide. Each floating gate includes a tunneling arm that extends over the cell""s source line and is separated therefrom by thin tunnel oxide. A word line is formed over the floating gates of the storage cells in each row of the array. The word line is separated from the underlying floating gate by a layer of oxide/nitride/oxide (ONO). The word lines run perpendicular to the buried n+ bit lines and extend over a second portion of the channel region of each cell in the row to define the internal access transistor of the cell. The word line is separated from the second portion of the channel region by the ONO layer.
Operation of a flash memory cell is taught by Hong in U.S. Pat. No. 5,349,220 where a semiconductor device is formed on a substrate lightly doped with a dopant. A source region and a drain region are formed in the substrate on the surface thereof. A dielectric layer is deposited upon the substrate. A floating gate electrode is formed on the dielectric layer proximate to at least the edges of these source region and the drain region. Additional dielectric material is deposited on the surface of the floating gate electrode, and a gate electrode is deposited upon the surface of the additional dielectric material.
A different flash memory cell and a method of forming the same are proposed by Kim in U.S. Pat. No. 5,702,965. According to Kim, program and erasure capabilities of the cell can be improved by forming an insulation spacer of an ONO or ON structure formed at the sidewalls of the floating gate. This structure also prevents the reduction of the coupling ratio and leakage electrons through the floating gate and the control gate. Still another process for making a high density split gate nonvolatile memory cell is described by Hsia, et al., in U.S. Pat. No. 4,861,730 where the drain region is self-aligned to the floating gate and the source region is self-aligned to the control gate. Fully self-aligned implantation is made possible by the process and structure using self-aligned etch.
While prior art offers different approaches for forming different split-gate flash memory cells, none of the cited methods show methods of increasing coupling ratio by employing a sidewall coupling that is claimed later in the embodiments of this invention, nor do they address the problem of punchthrough or junction break-down when increase in coupling ratio is attempted by increasing the size of the source region. It is well-known in the prior art that, as shown in FIG. 1B, of which FIG. 1A is a cross-section, the injection of carriers (80) into the floating gate (40) through edge (41) with voltage across source (25) and floating gate (40) can cause punch-through and junction break-down. The present invention makes possible to share the voltages with an additional poly line over the source region so that not only the excessive voltage levels are reduced, but at the same time the coupling ratio is increased with increased vertical area, but without increasing the lateral dimensions of the split-gate flash memory cell.
It is therefore an object of this invention to provide a method of forming split-gate flash memories with improved program speed.
It is another object of this invention to provide a method of forming split-gate flash memories with increased capacitive coupling between the source and the floating gate of the memory cell.
It is yet another object of the present invention to provide a method of forming split-gate flash memories with reduced punch-through and junction break-down attributes.
It is still another object of this invention to provide a split-gate flash memory cell having reduced size, increased capacitive coupling and improved program speed.
These objects are accomplished by providing a silicon substrate having a plurality of active and field areas defined; forming a gate oxide layer over said substrate; forming a first polysilicon layer over said [first] gate oxide layer; forming a nitride layer over said first polysilicon layer; forming a first photoresist layer over said nitride layer; patterning said first photoresist layer to define cell area and a floating gate[s]; etching said nitride layer through [said patterning in] said first photoresist layer and forming openings in said nitride layer reaching said first polysilicon layer; removing said first photoresist layer from said substrate; performing thermal oxidation of said first polysilicon layer exposed in said openings reaching said first polysilicon layer to form regions of poly-oxide; removing said nitride layer; etching said first polysilicon layer to form said floating gate using said regions of poly-oxide as a hard mask; then forming a second photoresist layer over said substrate and patterning to define source region; then performing ion implantation through openings in said second [patterned] photoresist layer to form a source region within said substrate; then removing said second photoresist layer; growing intergate oxide layer over said first polysilicon layer; forming second polysilicon layer over said intergate oxide layer; patterning said second polysilicon layer to form a control gate over said floating gate and [a] an additional third polysilicon line (poly-line) continuous over said source region; forming oxide spacers on sidewalls of said control gate and on sidewalls of said polyline; and performing ion implantation to form drain regions defined by said oxide spacers.
These objects are further accomplished by providing memory cell having an additional poly line with an extended vertical wall to share the voltage levels between the source and the floating gate in order to reduce punch-through and junction break-down and thereby to increase the coupling ratio and hence the program speed of the split-gate flash memory cell.